11.0 SOILS, GEOLOGY, AND SEISMICITY This chapter summarizes existing geologic conditions in the project area, describes applicable regulations, and evaluates project-related impacts associated with on-site geology, soils, seismic hazards, and slope stability. Mitigation measures are recommended as necessary to reduce significant geologic impacts. As described in Chapter 1.0, “Introduction,” the proposed project would not result in the loss of any known mineral resources, nor would it impede or interfere with mineral extraction operations, and the project area is not delineated as a locally important recovery site. Therefore, implementation of the proposed project would have no effect with regard to mineral resources, and this topic will not be discussed further in this chapter. 11.1 ENVIRONMENTAL SETTING 11.1.1 PHYSIOGRAPHIC SETTING The project area is located along the western slope of the Sierra Nevada Geomorphic Province. The Sierra Nevada Geomorphic Province is a tilted fault block nearly 400 miles long. Its east face is a high, rugged multiple scarp, in contrast with the gentle western slope, which disappears under sediments of the Great Valley. Deep river canyons are cut into the western slope. Their upper courses, especially in massive granites of the higher Sierra Nevada, are modified by glacial sculpturing, forming such scenic features as the Yosemite Valley. The high crest culminates in Mount Whitney, with an elevation of 14,495 feet above sea level near the eastern scarp. The metamorphic bedrock contains gold-bearing veins in the northwest trending Mother Lode. The northern Sierra Nevada boundary is marked where bedrock disappears under the Cenozoic volcanic cover of the Cascade Range (California Geological Survey 2002). The western slope of the Sierra Nevada is underlain by a series of metamorphic rock assemblages that trend north- northwest to south-southeast between the Mesozoic granitics of the Sierra Nevada batholith on the east and the sediment-filled Sacramento Valley to the west. These metamorphic rocks were developed by convergent plate tectonics in the early Paleozoic to Late Jurassic (400–120 million years ago) and consist of three northerly trending units bounded by faults and classified on the basis of age and lithology: the Eastern, Central, and Western metamorphic terranes. 11.1.2 LOCAL GEOLOGY The proposed trail alignment is located in the U.S. Geological Survey (USGS) Auburn and Greenwood 7.5- minute quadrangles and is approximately 14.2 miles from the confluence of the North Fork/Middle Fork American River confluence. The average gradient for most of the slopes along the proposed trail alignment is about 70% or flatter. Some segments, however, intersect slopes approaching 100% gradient. The proposed trail alignment would be approximately 200–400 feet above the river on the southern slope of the North Fork American River canyon. Published geologic maps show the majority of the proposed trail alignment in a mixed-rock area described as a “mélange belt,” composed of intermixed Mesozoic metasedimentary and metavolcanic rock with isolated bodies of limestone. The northeast section of the proposed trail alignment (from the Ponderosa Bridge to approximately 1 mile downstream) is mapped as Mesozoic metavolcanic flow rocks of the Logtown Ridge Formation (predominantly metamorphosed breccias, flows, and pyroclastic rocks) and Mesozoic metasedimentary rocks of the Mariposa Formation (predominantly slate, metagraywacke, and metaconglomerate) (Blackburn Consulting 2006, 2007). The entire project area is mapped as predominantly hard, fractured, metavolcanic flow rocks. This rock structure strikes generally north-northwest, with fracture/foliation planes dipping steeply northeast. The lesser bands are mapped as northwesterly trending metavolcanic tuffs and metashale that cross sections of the proposed trail alignment. These bands tend to be intensely weathered/foliated and, in general, less competent than rock flow. North Fork American River Trail Project Draft EIR EDAW Placer County 11-1 Soils, Geology, and Seismicity Isolated limestone bodies, typically hard and massive, are also mapped in the metavolcanic flow rock (Blackburn Consulting 2006, 2007). 11.1.3 RECREATIONAL GEOLOGIC FEATURES Recreational geologic resources typically include volcanoes, surface hydrothermal features, or surface expression of geologic features unique enough to generate recreational interest in the general public (e.g., natural bridges, caves, features associated with glaciation, and geomorphic features such as waterfalls, cliffs, canyons, and badlands). Based on a review of available geological literature, topographic maps, and a field visit to the site, there are no known recreational geologic resources in the project area. 11.1.4 SOIL RESOURCES Maps provided by the U.S. Soil Conservation Service (now called the Natural Resources Conservation Service) (SCS 1980) were reviewed to identify the distribution of soil types in the project area. Exhibit 11-1 provides a detailed map of the surficial soils in the project area. The physical and chemical characteristics of each soil type identified from the project site are presented below. 121 Auburn–Sobrante–Rock outcrop complex, 275% slopes—This soil is on undulating to very steep foothills. Rock outcrops are common. The soil forms in material weathered from metabasic or metasedimentary rock such as amphibolite schist, greenstone schist, or diabase at elevations of 125–3,000 feet. Soil is shallow over fractured, vertically tilted metabasic rock with rock outcrops. There are typically 4 inches of silt loam over 20 inches of silt loam subsoil. This soil is well drained, with slow to very rapid runoff and moderate permeability. This soil is used mostly for annual rangeland with small areas used for irrigated pasture. Rock is erosion resistant. 126 Boomer–Rock outcrop complex, 5–30% slopes—This soil forms over weathered metavolcanic (greenstone) bedrock at elevations of 500–5,000 feet. Soil is shallow over weathered schist and slate with rock outcrops. There are typically 10 inches of gravelly loam over 46 inches of gravelly clay loam subsoil. This soil is found on uplands; it is well drained, has slow to very rapid runoff, and has moderately slow permeability. This soil is used often for forestry and watersheds. Rock is erosion resistant. 164 Mariposa-Josephine complex, 5–30% slopes—The Mariposa-Josephine complex is encountered at elevations of 1,500–4,000 feet. Mariposa is common to ridges and south- and west-facing slopes, while Josephine is common to north- and east-facing slopes. Soil is shallow over weathered schist and slate with isolated rock outcrops. There are typically 6 inches of gravelly loam over 22 inches of gravelly clay loam subsoil. The complex is well drained with moderately slow permeability and moderate to high erosion hazard. Rock is erosion resistant. 167, 168 Mariposa–Rock outcrop complex, 50–70% slopes—This soil occurs on undulating to steep mountains at elevations of 1,600–5,600 feet. It is formed in material from metamorphosed sedimentary rocks. Soil is shallow over fractured, vertically tilted schist and slate with rock outcrops. There are typically 6 inches of gravelly loam over 28 inches of gravelly loam subsoil. This soil is well drained, has slow to very rapid runoff, and has moderate permeability. Timber production is the predominant use, with some grazing and deciduous fruit orchards. Rock is erosion resistant. 170 Maymen–Rock outcrop complex, 5–100% slopes—The Maymen–Rock outcrop complex occurs at elevations of 1,200–3,500 feet and generally consists of 50% Maymen soil, 20% Rock outcrop, 25% Mariposa gravelly loam, and 5% Josephine loam. Soil is shallow, with rock outcrops. There are typically 2 inches of gravelly loam over 10 inches of gravelly loam subsoil. The Maymen is a shallow, gravelly loam that is somewhat excessively drained, and permeability is moderate. Timber production and residential development are limited on this complex because of the slope, shallowness, and rock outcroppings. Rock is erosion resistant. EDAW North Fork American River Trail Project Draft EIR Soils, Geology, and Seismicity 11-2 Placer County Source: United States Department of Agriculture Soil Conservation Service 1977 Soil Types in the Project Area Exhibit 11-1 North Fork American River Trail Project Draft EIR EDAW Placer County 11-3 Soils, Geology, and Seismicity 190 Sites–Rock outcrop complex, 15–30% slopes—This soil occurs on mountains at elevations of 600–5,000 feet. It is formed in material weathered from metabasic and metasedimentary rocks. This soil is well drained, has slow to very rapid runoff, and has moderately slow permeability. Soil is shallow, with rock outcrops. There are typically 16 inches of loam and clay over 50 inches of clay (weathered rock). Timber production is the predominant use, with some areas cleared and used for deciduous fruit orchards. Rock is erosion resistant. SHRINK-SWELL POTENTIAL Shrink-swell potential is the potential for volume change in a soil with a loss or gain in moisture; soils swell when wet and shrink when dry. If the shrink-swell potential is rated moderate to high, volume changes can eventually result in damage to subsurface structures if the structures are not designed and constructed appropriately to resist the changing soil conditions. Soils with high clay content tend to be most affected by shrink and swell. The potential for soil to undergo shrink and swell is greatly enhanced by the presence of a fluctuating, shallow groundwater table. Volume changes of expansive soils can result in the consolidation of soft clays after the water table drops or fill is placed. NATURALLY OCCURRING ASBESTOS Asbestiform minerals occur naturally in rock and soil as the result of natural geologic processes, often in veins near earthquake faults in the Coast Range and the foothills of the Sierra Nevada. Naturally occurring asbestos can take the form of long, thin, separable fibers. Natural weathering or human disturbance can break naturally occurring asbestos down to microscopic fibers, easily suspended in air. There is no health threat if asbestos fibers in soil remain undisturbed and do not become airborne.